This application is concerned with the mechanisms by which usage alters synaptic operation. Brief periods (1 sec) of high frequency stimulation of hippocampal pathways induce an extremely stable enhancement of transmission through the synapses generated by that pathway. This effect, long-term potentiation ("LTP") has been followed for weeks in rats implanted with chronic electrodes. The proposal is concerned with the substrates of LTP and the cellular processes which produce those substrates. Previous work has shown that LTP induced in the projection from the regio inferior to regio superior (the Schaffer-commissural projections) is specific to the pathway receiving high frequency stimulation and that other inputs to the same dendritic target regions are never potentiated. This result combined with data indicating that high frequency stimulation does not alter the properties of the Schaffer-commissural axons indicates that LTP very likely is due to a modification of synapses (or of their immediate environment). Investigations into the nature of this hypothesized modification have uncovered two possibilities: 1) high frequency stimulation causes an increase in the number of synapses formed on the shafts of the dendrites as well as an apparent change in the shape of the dendritic spines; and 2) an increase in the number of sodium independent glutamate binding sites in hippocampal membranes. It is possible that these effects are linked either directly (i.e., shape change causes the appearance of new sites) or through the cellular machinery responsible for them. Much of the proposed work is intended to refine the measurements of these structural and chemical effects and to test for the possibility that they are the substrates of long-term potentiation. Therefore the time course for the two groups of effects will be established as well as their reaction to experimental manipulations which block the induction of LTP. With regard to those cellular processes which are triggered by the high frequency stimulation and which produce the substrates of LTP, previous work has shown that "potentiating" stmulation causes a rapid and selective change in the endogenous phosphorylation of a protein with an apparent molecular weight of 40,000 daltons. We have identified this protein to be the alpha subunit of pyruvate dehydrogenase. We have also found that the pyruvate supported calcium uptake by brain mitochondria is markedly affected by the phosphorylated state of the alpha unit and by pyruvate dehydrogenase activity. These observations have led us to propose that repetitive stimulation somehow induces the phosphorylation of Alpha-PDH which in turn suppresses calcium uptake by mitochondria, thereby resulting in elevated intracellular calcium. The proposed studies will attempt to test this hypothesis by nmeasuring the effects of high frequency stimulation on PDH activity and calcium uptake by mitochondria.